Production printing systems often comprise multiple printheads to increase speed and/or improve print quality. For example, inkjet printheads, electrophotographic toner printheads, wax printheads, etc., have inherent technological limits pertaining to the volume of colorant (e.g., ink, toner, etc.) dispersed over time. Thus multiple printheads may be employed to quickly generate a particular image and/or apply a large volume of colorant over a given period of time to improve print quality (i.e., image optical density).
To meet the demand for producing relatively high quality images, printers are designed with a number of printheads in a fixed array arrangement, wherein only the media is in motion. This is a technique commonly used in high speed production inkjet printers. However, due to the technical limitations, the design of fast operating printheads is difficult to accomplish. An alternate way of producing high color image quality is to increase the number of printheads and print an image multiple times on the media at same location. This mode of printing is usually referred as “multiple pass” printing. Analogously, printing with a single printhead is referred to as “single pass” printing. As a consequence of multipass printing, integrating twice the number of printheads, subsequently results in an image being printed at double the speed.
A specific type of “dual-pass printing” design uses two channels to print the same data twice on a media with relatively fast motion. These two passes use a different set of screens/halftones to print. Furthermore, this design also allows for a controllable switching mechanism between single pass printing and dual-pass printing. The screens/halftones for single pass printing are usually calibrated to produce 256 distinct output “gray” levels on the media. However, when two single passes print an image twice with the aforementioned calibrated screens/halftones, the resultant output does not produce the desired set of 256 distinct gray scale levels. In addition, the gray levels pertaining to shadow tone regions may be saturated with colorant and at the point in the tone range where saturation occurs, the solid area density ceases to increase. Moreover, print irregularities and artifacts may be caused by variations in fixed print-head array arrangements due to print-head overlaps and physical variations between print-heads. Calibration is therefore employed to produce 256 distinct gray scale levels when printing using a dual-pass mechanism.
Calibration of printing systems, particularly production printing systems, often includes a significant amount of manual intervention and time to achieve desired results. Additionally, any calibration process designed to compensate print non-uniformities is often dependent on the accurate detection of non-uniformities. Thus, if the detection protocol of a calibration process is inaccurate, then the entire calibration process is generally inaccurate. Consequently, there is a need for an automated calibration procedure involving compensation of print non-uniformities and artifacts to relatively satisfactory levels where the Human Visual System (HVS) perceives the printed output as uniform.